Exhaust system for three cylinder engine

ABSTRACT

Disclosed herein is an exhaust gas passage system for an outboard motor having an internal combustion engine with three vertically aligned cylinders extending with their axes in the fore and aft direction, which exhaust gas passage system advantageously occupies a minimum of width and includes three branch passages which respectively extend forwardly from the three exhaust ports and along one side of the engine, which merge into a forwardly located common duct or exhaust gas passage, and which have equal acoustical lengths between all of the cylinders and the common exhaust gas passage.

United States Patent [191 Ziegler Nov. 20, 1973 EXHAUST SYSTEM FOR THREE CYLINDER ENGINE [75] Inventor: Kurt F. Ziegler, Zion, ll].

[73] Assignee: Outboard Marine Corporation,

Waukegan, Ill.

[22] Filed: July 18, 1972 [21] Appl. No.: 272,762

[52] US. Cl 60/313, 60/314, 60/323, 115/ 17 [51] Int. Cl. F0ln 7/10, B63h 21/26 [58] Field of Search 115/17; 60/312, 313, 60/314, 323, 273

[56] References Cited UNITED STATES PATENTS 3,543,509 12/1970 Boerrna 60/323 3,692,006 9/1972 Miller 60/273 3,695,238 10/1972 Boerma 60/313 FOREIGN PATENTS OR APPLICATIONS 966,932 9/1957 Germany 60/313 Primary ExaminerDouglas l-lart Attorney'Robert E. Clemency et a1.

[5 7] ABSTRACT Disclosed herein is an exhaust gas passage system for an outboard motor having an internal combustion engine with three vertically aligned cylinders extending with their axes in the fore and aft direction, which exhaust gas passage system advantageously occupies a minimum of width and includes three branch passages which respectively extend forwardly from the three exhaust ports and along one side of the engine, which merge into a forwardly located common duct or exhaust gas passage, and which have equal acoustical lengths between all of the cylinders and the common exhaust gas passage.

13 Claims, '10 Drawing Figures EXHAUST SYSTEM FOR THREE CYLINDER ENGINE BACKGROUND OF THE INVENTION The invention relates generally to marine propulsion devices and more particularly to such marine propulsion devices which include a vertically aligned series of three cylinders. Still more particularly, the invention relates to exhaust gas passage systems for three cylinder outboard motors.

Outboard motors with three vertically aligned cylinders are known. One such outboard motor is disclosed in the U.S. Boerma Pat. No. 3,543,509, issued Dec. 1, 1970, which patent discloses an outboard motor having three vertically aligned cylinders and an exhaust gas system including a baffle for equalizing back pressure on the three cylinders to provide smooth idling operation Attention is also directed to the U.S. Miller Pat. No. 3,520,270, issued July 14, 1970.

SUMMARY OF THE INVENTION The invention provides an exhaust gas passage system for an outboard motor including an internal combustion engine with three vertically aligned cylinders extending with their axes in the fore and aft direction, which exhaust gas passage system extends forwardly along one side of the engine block from the ends of a vertically aligned and laterally extending series of exhaust ports, which occupies a minimum of width, and which includes three branch passages extending respectively from the three exhaust ports and merging into a common duct or exhaust gas passage to provide equal acoustical lengths between each of the cylinders and the common exhaust gas passage. The passages are also designed to have substantially equal flow areas defined by smoothly merging wall surfaces to avoid creating turbulence in the exhaust gas flow.

More specifically, and in further accordance with the invention, the exhaust gas passage system includes a plurality of interconnecting passages or passage portions which, except for openings which are provided in a manifold plate and which could be considered extensions of the exhaust ports, are generally in co-planar relation to each other and which extend forwardly from a laterally extending series of vertically aligned exhaust ports. In addition, in the illustrated embodiments and particularly with respect to modification of previously existing engines, the common exhaust gas passage has a portion which extends horizontally at a level below the lower exhaust port for communication with a side inlet port in the engine block.

Further in this regard, the exhaust gas passage system is formed through cooperation between the engine or cylinder block and a manifold plate and an exhaust manifold which are mounted on one side face of the engine block in overlying relation to each other and which provide cooperating wall means defining the exhaust gas passage system. In addition, a manifold cover is mounted on the exhaust manifold to provide a water jacket communicating through ducts in the exhaust manifold and in the manifold plate with other portions of the engine cooling jacket.

One of the principal objects of the invention is the provision of an exhaust gas system for a three cylinder engine having vertically aligned and spaced exhaust ports, which exhaust gas system occupies a minimum of space and provides acoustical flow paths of equal lengths from each of the cylinders to a common exhaust gas passage.

Another of the principal objects of the invention is the provision of an exhaust gas system which is as outlined in the preceding paragraph, which is adapted for association with an engine block including an exhaust gas inlet port below the lower exhaust port, and which includes a common exhaust gas passage communicating with the exhaust gas inlet port in the engine block.

Another of the principal objects of the invention is the provision of an exhaust gas passage system which is located on one side of an engine block and which is relatively thin in width so as to facilitate construction of an outboard motor with a slim transverse profile.

Another of the principal objects of the invention is the provision of an exhaust gas passage system which is arranged to cooperate with a series of vertically aligned and laterally extending exhaust ports and which is principally formed at a right angle to the lateral projection of the exhaust ports.

Other objects and advantages of the invention will become known by reference to the following description, claims, and drawings.

DRAWINGS FIG. 1 is a side elevational view, partially broken away and in section, of an outboard motor embodying various of the features of the invention.

FIG. 1A is a side elevational view, partially broken away and in section, of an outboard motor which is a modification of a previously existing engine and which incorporates various of the features of the invention.

FIG. 2 is an enlarged fragmentary view, partially in section, taken along a horizontal plane through the engine block of the outboard motor shown in FIGS. 1 and 1A.

FIG. 3 is a fragmentary, enlarged sectional view taken along a vertical fore and aft plane through the outboard motor shown in FIG. 1.

FIG. 3A is a fragmentary, enlarged sectional view taken along a vertical fore and aft plane through the outboard motor shown in FIG. 1A.

FIG. 4 is an exploded view of two mating surfaces embodied in the outboard motor shown in FIG. 1.

FIG. 4A is an exploded view of two mating surfaces embodied in the outboard motor shown in FIG. 1A.

FIG. 5 is an exploded view of two other mating surfaces embodied in the outboard motor shown in FIG. 1.

FIG. 5A is an exploded view of two other mating surfaces embodied in the outboard motor shown in FIG.

FIG. 6 is a sectional view taken along line 6-6 of FIG. 4.

Before explaining the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts set forth in the following general description or illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

GENERAL DESCRIPTION Shown in FIGS. 1 and 1A of the drawings are outboard motors 11 and 11A which embody various of the features of the invention. The outboard motor 11 illustrates a preferred embodiment of the invention while the outboard motor 11A illustrates modification of a previously existing engine to incorporate various of the features of the invention. As the outboard motors 11 and 11A are generally of similar construction, only the outboard motor 11 will be described in detail, except as noted below. All structure of the outboard motor 11A which is similar to that in the outboard motor 11 is referred to in the drawings with the same numerals, except that a postscript A has been added.

The outboard motor 11 conventionally includes (See FIG. 1) a transom bracket 13 which is adapted to be connected to a boat hull (not shown) and which is connected through a transversely and horizontally extending tilt pin 15 to a swivel bracket 17 which, in turn, is connected through a vertically extending king pin 19 to a propulsion unit 21. The propulsion unit 21 conventionally includes a lower unit 23 including a drive shaft housing 25 which is normally at least partially submerged in water and which supports a propeller 27 operable in the water, together with a powerhead 29 which is supported on the drive shaft housing 25, and a plate or adaptor member 31 which intervenes between the powerhead 29 and the drive shaft housing 25.

The powerhead 29 comprises an engine 32 including an engine block 33 which defines three vertically aligned cylinders, i.e., an upper cylinder 35, an intermediate cylinder 37, and a lower cylinder 39, with the axes of the cylinders lying in a vertical plane extending fore and aft, i.e., in a vertical plane extending perpendicularly to the transversely and horizontally extending tilt pin 15. Each of the cylinders 35, 37, and 39 has located therein a piston 41 which is respectively connected through a piston rod 43 to a vertically extending crankshaft 45 which is ultimately drivingly connected to a propeller shaft 47 carrying the propeller 27. Connected to the rear face of the engine block 33 is a head member 49 (See FIG. 2) which supports three vertically aligned spark plugs 51. Formed in the engine block 33 is a suitable engine cooling water jacket 53.

Each of the upper, intermediate, and lower cylinders 35, 37, and 39 respectively communicates (See FIG. 3) with upper, intermediate, and lower exhaust ports 55, 57, and 59, respectively. In the illustrated construction, each of the exhaust tubes 55, 57, and 59 comprises three adjacent sub-ports 61 and a common exhaust gas duct 63 into which the sub-ports 61 merge. The upper, intermediate, and lower exhaust ports 55, 57, and 59 are located vertically above one another on the starboard or right side of the engine block 33 when looking from the front toward the rear and all terminate in a generally planar starboard side face 65 of the engine block 33.

Located in the adaptor plate 31 (See FIG. 3) is a vertically extending exhaust gas passageway 67 having an upper inlet 69 and a lower outlet 71. The lower outlet 71 communicates (See FIG. 1) with an exhaust gas box or housing 74 located in the drive shaft housing 25, surrounded by a waterjacket, and communicating through a suitable outlet and connected exhaust gas passageway 76 in the drive shaft housing 25 with an exhaust gas discharge passageway in the propeller 27. If desired, a tuning tube 73 can communicate with the outlet 71 so as to optimize operation at a particular speed.

In the outboard motor 11A shown in FIGS. 1A and 3A, the lower outlet 71A communicates with an exhaust gas tube 73A which is located in the drive shaft housing 25A and can be arranged and constructed generally as shown in the US. Miller Pat. No. 3,520,270, issued July 14, I970, and incorporated herein by reference. The exact details of the arrangement for discharging exhaust from the lower units 23 and 23A are not a part of the invention and, accordingly, will not be further described.

The exhaust gas passageway 67 in the adaptor plate 31 receives exhaust gas from a smooth walled arcuately curving exhaust gas conduit which extends in the engine block 33 and which terminates in the bottom face 77 of the engine block 33 and with an outlet port 79 mating with the inlet 69 of the exhaust gas passageway 67 in the adaptor plate 31. The exhaust gas conduit 75 also terminates with an inlet port 81 located in the starboard side face 65 of the engine block below the exhaust port 59 and in general vertical alignment with the exhaust ports 55, 57, and 59 (See the right side of FIG. 4).

In particular accordance with the invention, there is provided (See FIG. 3) an exhaust gas passage means or an exhaust gas passage arrangement 101 for connecting the three exhaust ports 55, 57, and 59 of the engine to the inlet port 81 of the exhaust gas conduit 75 in the engine block 33 in such manner that the acoustical length of the path of travel of exhaust gases from each of the cylinders 35, 37, and 39 to the exhaust gas conduit inlet port 81 is uniform and in such manner as to occupy a minimum of space and thereby so as to avoid imparting a bulky appearance to the outboard motor 11. The paths of travel are designed to afford equal mass flow and are therefore of generally the same cross sectional area throughout. In addition, the walls defining the paths are smooth and designed to minimize any abrupt change in direction and so as to avoid turbulence in the exhaust gas flow induced by directional changes.

In further accordance with the invention, the illus trated exhaust gas passage means or arrangement 101 includes (See FIG. 3) an exhaust manifold 103 which is bolted or otherwise connected to the starboard side face 65 of the engine block 33. Preferably a manifold plate 105 is sandwiched between the exhaust manifold 103 and the starboard side face 65 of the engine block 33. The outer surface of the exhaust manifold 103 is preferably enclosed by a plate or cover 107 which, in cooperation with the exhaust manifold 103, defines a water jacket 109 which communicates, through porting in the exhaust manifold 103 and manifold plate 105, with other portions of the engine cooling water jacket 53. The manifold cover 107 can be connected to the exhaust manifold 103 by the same bolts which connect the exhaust manifold 103 and manifold plate 105 to the starboard face 65 of the engine block 33.

Shown to the right in FIG. 4 is the starboard face 65 of the engine block 33. Shown to the left in FIG. 4 is the inner surface of the manifold plate 105, which inner surface mates with the starboard face 65 of the engine block 33. Shown to the right in FIG. 5 is the outer surface of the manifold plate 105 as it appears when mounted on the engine block 33. Shown to the left in FIG. 5 in the inner surface of the exhaust manifold 103,

which inner surface mates with the outer surface of the manifold plate 105. I

The exhaust gas passage means or arrangement 101 includes formation of the engine block 33 so that, as shown in FIG. 3 and to the right in FIG. 4, the sub-ports 61 of the upper, intermediate, and lower exhaust ports 55, 57, and 59, respectively, are all generally of the same construction. In addition, the exhaust ducts 63 leading from the sub-ports 61 of the upper and lower cylinders 35 and 39 are generally of the same size. The exhaust duct 63 leading from the sub-ports 61 of the intermediate cylinder 37 is somewhat enlarged and extends upwardly and forwardly, as shown at 111 in FIGS. 3 and 4, to facilitate exhaust gas mass flow through the intermediate exhaust port 57. Also, the exhaust duct 63 leading from the intermediate cylinder is partially defined by a deflector 112.

The exhaust gas passage means or arrangement 101 also includes (See the right-hand side of FIG. 5) wall means on the outer surface of the manifold plate 105 and (See the left-hand side of FIG. 5) on the inner surface of the exhaust manifold 103 defining an exhaust gas passage system having opposed inner and outer side walls 122 and 124 formed respectively in the exhaust plate 105 and exhaust manifold 103 and including (See FIG. 5) a common exhaust passage 121 which is defined by inner and outer transverse walls 123 and 125, respectively, and which includes a generally vertical extending portion 127 located forwardly of the lower exhaust port 59 and having an upper end located forwardly of and at about the top of the lower exhaust port 59. The common passage 121 also includes a forwardly and rearwardly extending horiziontal discharge portion 129 which communicates through a curved portion 131 with the lower end of the vertical portion 127 and which terminates in a side discharge opening or outlet 133 located to afford lateral exhaust gas discharge through the inlet port 81 in the starboard face 65 of the engine block 33 and into the exhaust gas conduit 75 in the engine block 33.

The inner side wall 122 of the exhaust gas passage system is formed (See the right-hand side of FIG. 5) in the outer surface of the manifold plate 105 and is generally planar, except for short incline ramps 135, 137,

" inner surface of the exhaust manifold 103 and is also generally planar except for an inclined or curved portion 143 opposite the side discharge passage port or outlet 133 and except for ramps 0r inclines 145 and 147 located opposite to the lower ends of the intermediate and lower exhaust ports 57 and S9, and except for a ramp or inclined surface 148 located opposite to the upper end of the upper exhaust port 55 and except for a more or less continuously extending fillet 149 between the outer side wall 124 and the transverse walls which define the exhaust gas passageway system.

The exhaust gas passage system also includes a first or lower branch passage 151 which is formed (See FIG. 5) by upper and lower transverse walls 153 and 155, respectively, and which includes a first portion 157 extending arcuately upwardly and rearwardly from the upper end of the vertically extending passage portion 127 and a second portion 159 which curves rearwardly and downwardly from the first portion 157 and then forwardly and communicates (See FIG. 3 and the left side of FIG. 4) through a lower opening 161 in the manifold plate with the lower exhaust port 59.

The exhaust gas passage system also includes an extension passage 163 which is formed by an upward continuation of the transverse walls 123 and 125, which extends vertically from the upper end of the vertical portion 127 of the common passage 121 and forwardly of the exhaust port 57 and which includes an upper end located forwardly of and at about the top of the intermediate exhaust port 57.

Still further, the exhaust passage system includes a second or intermediate branch passage 165 formed (See FIG. 5) by upper and lower transverse walls 167 and 169, respectively, and including a first portion 171 which extends from the upper end of the extension passage 163 and which curves upwardly and rearwardly and a second portion 173 which extends from the first portion 171 and arcuately downwardly and rearwardly and then somewhat forwardly to communicate with the intermediate exhaust port 57 through an intermediate opening 175 in the manifold plate 105 extending generally above the intermediate exhaust port 57 and having (See the right side of FIG. 5) a lower portion 177 directly transversely communicating with approximately the upper one-third of the area of the exhaust sub-ports 61.

It is noted thatrin the outboard motor 11A, the second branch passage portions 159A and 173A curve downwardly and rearwardly but do not thereafter turn forwardly and that the lengths of the flow paths from the cylinders 35A, 37A, and 39A to the common pas sage 121A is approximately equal.

The exhaust gas passage system further includes a third or upper branch passage 181 which is principally formed by an upper transverse wall 183 and which includes a first portion 185 extending arcuately upwardly and rearwardly from the upper end of the extension passage 163 and a second portion 187 which extends from the first portion 185 upwardly and rearwardly to an upper opening 189 in the manifold plate 105 in general registry with the upper exhaust port 55. The upper and forward end of the upper transverse wall 167 of the second branch passage 165 is reversely curved and merges in the area immediately forwardly of and slightly above the bottom of the upper exhaust port 55 into a transverse bottom wall 191 for the third branch passage 181.

As indicated above, the upper and lower exhaust ports 55 and 59 are in general registry with the upper and lower openings 189 and 161 in the manifold plate 105 while the intermediate exhaust port 57 is only in partial registry with the manifold plate opening 175. Accordingly, in order to provide adequate communication between the second branch passage 165 and the intermediate exhaust port 57, the inner surface of the manifold plate 105 is provided (See FIG. 3 and the left side of FIG. 4) with a recess 193 which extends downwardly from the opening 175 and in registry with the remainder or lower portion of the intermediate exhaust port 57.

The upper portion of the recess 193 is laterally increased in depth to accommodate mass flow from the bottom portion thereof by reason of a laterally outwardly and upwardly curving wall 195 (See FIGS. 3 and 6) having an upper margin which defines the bottom of the intermediate opening 175 when seen in horizontal view from the outside (See the left side of FIG. 4 and the right side of FIG. 5).

Included in both the manifold plate 105 and in the exhaust manifold 103 are cooling water ports 201 connecting the manifold water jacket 109 with other portions of the engine cooling jacket 53.

The particularly illustrated arrangement of the exhaust gas passage system is such that the second and third branch passages 165 and 181, respectively, have equal acoustical lengths from the intermediate and upper cylinders 37 and 35, respectively, to the upper end of the extension passage 163.

In addition, the first or lower branch passage 151 has an acoustical length, between the lower cylinder 39 and the upper end of the vertical portion 127 of the common exhaust passage 121, which is equal to the acoustical length of one of the second and third branch passages 165 and 181, plus the acoustical length of the extension passage 163 between the upper end of the vertical portion 127 of the common exhaust passage 121 and the second and third branch passages 165 and 181, respectively. As a consequence, the acoustical length from the upper end of the vertical portion 127 of the common exhaust passage 121 to each of the upper, intermediate, and lower cylinders 35, 37, and 39, respectively, is substantially equal. In addition, the vertical portion 127 of the common exhaust passage 121 and the extension passage 163 are located forwardly of and in the same plane as the branch passages 151, 165, and 181 which all initially extend upwardly and rearwardly.

The disclosed exhaust gas passage arrangement 101 provides acoustical paths of equal length in a very compact arrangement extending principally forwardly of and to the starboard side face of the exhaust ports 55, 57, and 59 in the starboard side face 65 of the engine block 33. Because the exhaust gas passage system 101 is oriented forwardly from the laterally extending exhaust ports 55, 57, and 59, the width of the engine and of the outboard motor 11 is not materially increased over or beyond prior constructions which do not include the advantageous feature of equal acoustical flow lengths between each of the cylinders 35, 37, and'39 and the common exhaust passage 121 of the exhaust gas passage system 101.

As shown in dotted outline in FIG. 1, the exhaust gas passage arrangement and specifically the common passage 121 could extend more or less directly downwardly to communicate with a passageway 80 (shown in dotted outline) which extends through the adaptor plate 31 and which would be located forwardly of the passage 67 so as to avoid employment of the horizontal portion 129 and the curved portion 131 of the common passage 121. Such communication would shorten the distance of exhaust gas flow prior to entry into the lower unit exhaust box 74 and would involve less change in direction and consequently improved exhaust gas flow.

The use of an exhaust gas passage system as disclosed herein also beneficially permits use of carburetors with fixed main and idle jets. In addition, the engine can be tuned to optimum performance at a given speed by proper dimensioning of the tuning tube 73. Either a conical or cylindrical tuning tube can be employed.

Optimum high speed performance can be obtained without a tuning tube by affording direct exhaust gas flow into the exhaust gas box 74 in the lower unit.

Various of the features of the invention are set forth in the following claims.

What is claimed is:

1. An outboard motor comprising a three cylinder engine block including a side face with an inlet port adjacent the bottom of said engine block and with upper, intermediate, and lower vertically spaced exhaust ports, said engine block also having a bottom face with an outlet port communicating through said engine block with said inlet port, a drive shaft housing supporting said engine block and including means mounting a propeller for rotation in response to engine operation, and exhaust gas passage means including an exhaust manifold connected to said side face of said engine block and including wall means defining a common passage communicating with said inlet port and having a vertically extending portion located forwardly of said lower exhaust port and including an upper end located forwardly of and at about the top of said lower exhaust port, a first branch passage extending from said upper end of said vertical portion of said common passage and communicating with said lower exhaust port, said wall means further defining a vertical extension passage extending upwardly from said upper end of said vertical portion of said common passage and having an upper end located forwardly of and at about the top of said intermediate exhaust port, said wall means also defining a second branch passage extending from said upper end of said extension passage and communicating with said intermediate exhaust port, said wall means further defining a third branch passage extending from said upper end of said extension passage and communicating with said upper exhaust port, said second and third branch passages having equal acoustical lengths from said intermediate and upper exhaust ports to said upper end of said extension passage, and said first branch passage having an acoustical length from said lower exhaust port to said upper end of said vertical portion of said common passage equal to the sum of the acoustical length of said extension passage, plus the acoustical length of one of said second and third branch passages.

2. An outboard motor in accordance with claim 1 wherein said first branch passage includes a first portion extending arcuately upwardly and rearwardly from said upper end of said vertical portion of said common passage and a second portion extending arcuately downwardly and rearwardly from said first portion.

3. An outboard motor in accordance with claim 1 wherein said second branch passage includes a first portion which extends arcuately upwardly and rearwardly from said upper end of said extension passage and a second portion which extends arcuately downwardly and rearwardly from said first portion.

4. An outboard motor in accordance with claim 1 wherein said third branch passage extends upwardly and rearwardly from said upper end of said extension passage.

5. An outboard motor in accordance with claim 1 wherein said second branch passage includes a portion which extends arcuately upwardly and rearwardly from said upper end of said extension passage and a second portion which extends arcuately downwardly and rearwardly from said first portion, wherein said third branch passage extends upwardly and rearwardly from said upper end of said extension passage, and wherein said third branch passage partly extends in common with said first portion of said second branch passage.

6. An outboard motor in accordance with claim 1 wherein said engine further includes a manifold plate located between said exhaust manifold and said side face of said engine block, said wall means also being formed by said manifold plate.

7. An outboard motor in accordance with claim 6 wherein said manifold plate includes lower, intermediate, and upper openings respectively communicating with said first, second and third branch passages and located in at least partial registry with said lower, intermediate, and upper exhaust ports.

8. An outboard motor in accordance with claim 7 wherein said upper and lower openings in said manifold plate are in full registry with said upper and lower exhaust ports.

9. An outboard motor in accordance with claim 7 wherein said manifold plate includes an exhaust passage port located below said openings and in communication with said inlet port in said engine block, and wherein said common passage terminates with a portion extending generally horizontally and communicating with said exhaust passage port in said manifold plate.

10. An outboard motor in accordance with claim 1 wherein said engine further includes an engine cooling jacket, a manifold cover connected to the outside of said exhaust manifold, a manifold cooling jacket defined by said exhaust manifold and said manifold cover,

and a cooling water duct extending throughsaid exhaust manifold and communicating with said manifold cooling jacket and with said engine cooling jacket.

11. An outboard motor in accordance with claim 1 wherein said exhaust gas passages extend in co-planar relation to one another.

12. An outboard motor comprising an engine with a cylinder block including means defining first, second, and third vertically aligned cylinders, a side face with an inlet port adjacent to the bottom of said engine block and with upper, intermediate, and lower vertically spaced and aligned exhaust ports respectively communicating with said cylinders and located above and in general alignment with said inlet port, said engine block also having a bottom face with an outlet port communicating through said engine block with said inlet port, and an engine cooling jacket, a drive shaft housing supporting said engine block and including means mounting a propeller for rotation in response to engine operation, a manifold plate connected to said side face of said engine block and including lower, intermediate, and upper openings respectively communicating with said lower, intermediate, and upper exhaust ports and an exhaust passage port located below said openings in communication with said engine block inlet port, an exhaust manifold connected to said manifold plate, a manifold cover connected to the outside of the exhaust manifold, a manifold cooling jacket defined by said exhaust manifold and said manifold cover, a cooling water duct extending through said exhaust manifold and manifold plate and communicating with said manifold cooling jacket and with said engine cooling jacket, and an exhaust gas passage system communicating between said exhaust ports and said inlet port in said engine block, said exhaust gas passage system including wall means defined by said manifold plate and said exhaust manifold and including a common passage having a discharge portion extending horizontally forwardly and rearwardly and communicating at the rearward end thereof with said exhaust passage port in said manifold plate and having a vertically extending portion communicating with said horizontal portion and located forwardly of said lower exhaust port and having an upper end located forwardly of and at about the top of said lower exhaust port, said wall means also including a first branch passage including a first portion extending arcuately upwardly and rearwardly from said upper end of said vertical portion of said common passage and a second portion extending arcuately downwardly and rearwardly from said first portion and communicating with said lower opening in said manifold plate, said wall means further including a vertical extension passage extending upwardly from said upper end of said vertical portion of said commonpassage and having an upper end located forwardly of and at about the top of said intermediate exhaust port, said wall means further including a second branch passage including a first portion which extends arcuately upwardly and rearwardly from said upper end of said extension passage and a second portion which extends arcuately downwardly and rearwardly from said first second branch portion and communicates with said intermediate opening in said manifold plate, said wall means further including a third branch passage extending partially in common with said first portion of said second branch passage and upwardly and rearwardly from said upper end of said extension passage and communicating with said upper opening in said manifold plate, all of said exhaust gas passages extending in generally coplanar relation to one another, said branch passages extending such that the acoustical lengths from said first and second cylinders to said upper end of said extension passage are equal and such that the acoustical length from said third cylinder to said upper end of said vertical portion of said common passage is equal to the sum of the acoustical length of said extension passage plus the acoustical length from one of said first and second cylinders to said upper end of said extension passage.

13. An outboard motor comprising an engine block including upper,'intermediate, and lower cylinders, and aside face with upper, intermediate, and lower vertically spaced exhaust ports respectively communicating with said upper, intermediate, and lower cylinders, a drive shaft housing supporting said engine block, and exhaust gas passage means extending parallel to said face and including an exhaust manifold connected to said side face of said engine block and including wall means defining a common passage located forwardly of said lower exhaust port and including a discharge outlet, said wall means also defining a first branch passage extending from said common passage and communicating with said lower exhaust port, said wall means also defining a vertical extension passage extending upwardly from said common passage, said wall means also defining a second branch passage extending from said extension passage and communicating with said intermediate exhaust port, said wall means also defining a third branch passage extending from said extension passage and communicating with said upper exhaust port, said second and third branch passages being formed so as to provide equal acoustical lengths from said intermediate and upper cylinders to said extension passage, and said first branch passage being formed so as to provide an acoustical length from said lower cylinder to said common passage equal to the sum of the acoustical length of said extension passage, plus the acoustical length of one of said second and third branch passages. 

1. An outboard motor comprising a three cylinder engine block including a side face with an inlet port adjacent the bottom of said engine block and with upper, intermediate, and lower vertically spaced exhaust ports, said engine block also having a bottom face with an outlet port communicating through said engine block with said inlet port, a drive shaft housing supporting said engine block and including means mounting a propeller for rotation in response to engine operation, and exhaust gas passage means including an exhaust manifold connected to said side face of said engine block and including wall means defining a common passage communicating with said inlet port and having a vertically extending portion located forwardly of said lower exhaust port and including an upper end located forwardly of and at about the top of said lower exhaust port, a first branch passage extending from said upper end of said vertical portion of said common passage and communicating with said lower exhaust port, said wall means further defining a vertical extension passage extending upwardly from said upper end of said vertical portion of said common passage and having an upper end located forwardly of and at about the top of said intermediate exhaust port, said wall means also defining a second branch passage extending from said upper end of said extension passage and communicating with said intermediate exhaust port, said wall means further defining a third branch passage extending from said upper end of said extension passage and communicating with said upper exhaust port, said second and third branch passages having equal acoustical lengths from said intermediate and upper exhaust ports to said upper end of said extension passage, and said first branch passage having an acoustical length from said lower exhaust port to said upper end of said vertical portion of said common passage equal to the sum of the acoustical length of said extension passage, plus the acoustical length of one of said second and third branch passages.
 2. An outboard motor in accordance with claim 1 wherein said first branch passage includes a first portion extending arcuately upwardly and rearwardly from said upper end of said vertical portion of said common passage and a second portion extending arcuately downwardly and rearwardly from said first portion.
 3. An outboard motor in accordance with claim 1 wherein said second branch passage includes a first portion which extends arcuately upwardly and rearwardly from said upper end of said extension passage and a second portion which extends arcuately downwardly and rearwardly from said first portion.
 4. An outboard motor in accordance with claim 1 wherein said third branch passage extends upwardly and rearwardly from said upper end of said extension passage.
 5. An outboard motor in accordance with claim 1 wherein said second branch passage includes a portion which extends arcuately upwardly and rearwardly from said upper end of said extension passage And a second portion which extends arcuately downwardly and rearwardly from said first portion, wherein said third branch passage extends upwardly and rearwardly from said upper end of said extension passage, and wherein said third branch passage partly extends in common with said first portion of said second branch passage.
 6. An outboard motor in accordance with claim 1 wherein said engine further includes a manifold plate located between said exhaust manifold and said side face of said engine block, said wall means also being formed by said manifold plate.
 7. An outboard motor in accordance with claim 6 wherein said manifold plate includes lower, intermediate, and upper openings respectively communicating with said first, second and third branch passages and located in at least partial registry with said lower, intermediate, and upper exhaust ports.
 8. An outboard motor in accordance with claim 7 wherein said upper and lower openings in said manifold plate are in full registry with said upper and lower exhaust ports.
 9. An outboard motor in accordance with claim 7 wherein said manifold plate includes an exhaust passage port located below said openings and in communication with said inlet port in said engine block, and wherein said common passage terminates with a portion extending generally horizontally and communicating with said exhaust passage port in said manifold plate.
 10. An outboard motor in accordance with claim 1 wherein said engine further includes an engine cooling jacket, a manifold cover connected to the outside of said exhaust manifold, a manifold cooling jacket defined by said exhaust manifold and said manifold cover, and a cooling water duct extending through said exhaust manifold and communicating with said manifold cooling jacket and with said engine cooling jacket.
 11. An outboard motor in accordance with claim 1 wherein said exhaust gas passages extend in co-planar relation to one another.
 12. An outboard motor comprising an engine with a cylinder block including means defining first, second, and third vertically aligned cylinders, a side face with an inlet port adjacent to the bottom of said engine block and with upper, intermediate, and lower vertically spaced and aligned exhaust ports respectively communicating with said cylinders and located above and in general alignment with said inlet port, said engine block also having a bottom face with an outlet port communicating through said engine block with said inlet port, and an engine cooling jacket, a drive shaft housing supporting said engine block and including means mounting a propeller for rotation in response to engine operation, a manifold plate connected to said side face of said engine block and including lower, intermediate, and upper openings respectively communicating with said lower, intermediate, and upper exhaust ports and an exhaust passage port located below said openings in communication with said engine block inlet port, an exhaust manifold connected to said manifold plate, a manifold cover connected to the outside of the exhaust manifold, a manifold cooling jacket defined by said exhaust manifold and said manifold cover, a cooling water duct extending through said exhaust manifold and manifold plate and communicating with said manifold cooling jacket and with said engine cooling jacket, and an exhaust gas passage system communicating between said exhaust ports and said inlet port in said engine block, said exhaust gas passage system including wall means defined by said manifold plate and said exhaust manifold and including a common passage having a discharge portion extending horizontally forwardly and rearwardly and communicating at the rearward end thereof with said exhaust passage port in said manifold plate and having a vertically extending portion communicating with said horizontal portion and located forwardly of said lower exhaust port and having an upper end located forwardly of and at about the top of said lower exhaust port, said wall means also including a First branch passage including a first portion extending arcuately upwardly and rearwardly from said upper end of said vertical portion of said common passage and a second portion extending arcuately downwardly and rearwardly from said first portion and communicating with said lower opening in said manifold plate, said wall means further including a vertical extension passage extending upwardly from said upper end of said vertical portion of said common passage and having an upper end located forwardly of and at about the top of said intermediate exhaust port, said wall means further including a second branch passage including a first portion which extends arcuately upwardly and rearwardly from said upper end of said extension passage and a second portion which extends arcuately downwardly and rearwardly from said first second branch portion and communicates with said intermediate opening in said manifold plate, said wall means further including a third branch passage extending partially in common with said first portion of said second branch passage and upwardly and rearwardly from said upper end of said extension passage and communicating with said upper opening in said manifold plate, all of said exhaust gas passages extending in generally co-planar relation to one another, said branch passages extending such that the acoustical lengths from said first and second cylinders to said upper end of said extension passage are equal and such that the acoustical length from said third cylinder to said upper end of said vertical portion of said common passage is equal to the sum of the acoustical length of said extension passage plus the acoustical length from one of said first and second cylinders to said upper end of said extension passage.
 13. An outboard motor comprising an engine block including upper, intermediate, and lower cylinders, and a side face with upper, intermediate, and lower vertically spaced exhaust ports respectively communicating with said upper, intermediate, and lower cylinders, a drive shaft housing supporting said engine block, and exhaust gas passage means extending parallel to said face and including an exhaust manifold connected to said side face of said engine block and including wall means defining a common passage located forwardly of said lower exhaust port and including a discharge outlet, said wall means also defining a first branch passage extending from said common passage and communicating with said lower exhaust port, said wall means also defining a vertical extension passage extending upwardly from said common passage, said wall means also defining a second branch passage extending from said extension passage and communicating with said intermediate exhaust port, said wall means also defining a third branch passage extending from said extension passage and communicating with said upper exhaust port, said second and third branch passages being formed so as to provide equal acoustical lengths from said intermediate and upper cylinders to said extension passage, and said first branch passage being formed so as to provide an acoustical length from said lower cylinder to said common passage equal to the sum of the acoustical length of said extension passage, plus the acoustical length of one of said second and third branch passages. 